B.O. Al-Bedoor

Modeling the coupled torsional and lateral vibrations of unbalanced rotors

Abstract A model for the coupled torsional and lateral vibrations of an unbalanced rotor is developed. The equations of motion, obtained using Lagrangian dynamics, showed inertial coupling and nonlinear interaction between the rotor torsional and lateral vibrations. A softening effect of the rotor torsional stiffness has appeared. This effect and some of the nonlinear interaction effects have appeared, for the first time in this model, due to treating the torsional angle as an individual motion. Simulation results showed energetic interaction between the rotor lateral and torsional vibrations. Responses at 1 X and 3 X in the torsional vibration signal and 1 X and 2 X in the lateral vibration signals have appeared.

Predicting the fatigue life of synchronous motor-driven compressor using the complex modal reduction technique

Abstract In this paper, the low cycle fatigue life for rotor systems driven by synchronous motors is predicted using the complex modal reduction technique. The system torsional model is derived using the lumping technique where, for accuracy, large number of stations is considered. The effect of bearing viscous damping is accounted for in the equations of motion. The Miner’s rule is adopted for calculating the commutative stresses for predicting the low cycle fatigue life for the full and the reduced-order models. The procedure is applied to an actual 19xa0000 hp synchronous motor driving a high-speed compressor. Simulation results showed excellent agreement in predicting the transient stresses between the full model and the 2-modes reduced model with vast reduction in computational time, i.e. around 90%. Moreover, the predicted fatigue life in terms of number of startups shows excellent agreement with a maximum error of about 4.2% in the predicted life.